Telephone equipment and networks which are widely available in many countries around the world provide mechanisms to enable a telephone connected to a central office telephone system ("CO") to establish a telephone connection with a telephone or other device connected to a different CO, the telephone connection being for use in transmitting voice, data, facsimile or other messages. Whenever one of the telephones involved in a telephone conversation is disconnected from its CO, that CO sends a signal to the other telephone to indicate that far-end disconnect has occurred. That signal is referred to as a far-end disconnect signal.
Far-end disconnect signals vary from country to country and from central office telephone system manufacturer to manufacturer. Some examples of such far-end disconnect signals include "busy signals," "dial tones," "turkey tones," "silence," and so forth. These signals typically have relatively narrow frequency ranges and typically have constant or cadenced signal power. For example, a cadence which is typically used to provide a far-end disconnect signal is characterized by approximately matched on and off periods. Finally, whenever a near-end party hears a far-end disconnect signal, he or she will understand that the far-end party has disconnected and, as a result, the near-end party will hang up his or her telephone.
A problem arises when the near-end party is not a person who can recognize a far-end disconnect signal, but is a machine or a computer processor which is part of an automated telephony application system. An example of such an automated telephony application system is a voice store and forward messaging system which records incoming telephone messages for playback to a system user at a later time, such as a ROLM.TM. PhoneMail.TM. voice store and forward messaging system manufactured by ROLM Systems of Santa Clara, Calif. Whenever a far-end telephone disconnects from a connection with a voice store and forward messaging system, the voice store and forward messaging system will receive a far-end disconnect signal. As one can readily appreciate, the voice store and forward messaging system needs to detect the far-end disconnect signal to prevent the voice store and forward messaging system from storing the far-end disconnect signal as if it were a portion of an incoming voice message.
In order to detect far-end disconnect signals and to distinguish them from other signals, a far-end disconnect signal detector must distinguish the characteristics of far-end disconnect signals, such as their power and/or frequency spectrum, from those of other input signals. Presently available automated telephony application systems are deficient in the methods they utilize to distinguish far-end disconnect signals. In particular, one type of presently available automated telephony application system utilizes a far-end disconnect signal detector which comprises a country-specific or manufacturer-specific frequency detector and a country-specific cadence detector to detect and to identify the characteristics of far-end disconnect signals which differ from country to country and from central office telephone system manufacturer to manufacturer. For example, a typical such country-specific far-end disconnect signal detector in Japan will utilize a narrow band filter which is centered at approximately 400 Hz as a frequency detector, whereas a similar frequency detector in Hong Kong will utilize two narrow band filters which are centered at 480 Hz and 620 Hz, respectively. In addition, the output from such a country-specific filter is transmitted to a country-specific cadence detector to identify country-specific cadences which characterize the country-specific far-end disconnect signal. Such a diversity of far-end disconnect signal characteristics results in higher costs for development, manufacturing and support of automated telephony application systems which are intended to be marketed throughout the world.
Another type of presently available automated telephony application system utilizes a far-end disconnect signal detector which comprises an input signal power detector that identifies either constant input signal power or cadenced input signal power. Such a signal power detector is deficient in that it often fails to distinguish certain human voice patterns that may produce constant or cadenced power for a short period of time. For example, when a person says "ahh . . . " for a few seconds continuously, a constant or a cadenced input signal power detector will mistakenly identify the voice as a far-end disconnect signal. This results in the automated telephony application system's disconnecting the telephone call erroneously, a condition generally known as "talk-off."
As a result of the above-described problems concerning the identification of far-end disconnect signals by automated telephony application systems which are presently available in the prior art, there is a need for a method and apparatus for detecting and identifying far-end disconnect signals without requiring country-specific or manufacturer-specific frequency or cadence detectors.